QVariant::save/load: unable to save type X

t3chNo

Hi, i use QListWidget in my gui. List widget items has data with type QObject* that points to my custom QObject derived class. When I drag and drop items, program outputs these lines:
QVariant::save: unable to save type 136.
QVariant::load: unable to load type 136.
The program has unexpectedly finished.
and crash. What would be the problem?

alexander

Can you describe data, which you insert in QListWidgetItem as QVariant?

t3chNo

A graphicsitem that inherits from QObject and QGraphicsPixmapItem.

alexander

you must register your data: @qRegisterMetaType(), qRegisterMetaTypeStreamOperators()@

alexander

are you insert graphicsitem as pointer?

t3chNo

Yes, as pointer. I cast QObject* when i set data.

t3chNo

I think i solved the problem. First, i look pointer classes like QPointer, QSharedDataPointer etc for stream operators that needed by qRegisterMetaTypeStreamOperators. I haven't found any. Then i set data as qint32. It works fine but storing pointers with qint32 type seems little weird.

t3chNo

Btw, item that has data with unregistered type and QAstractItemView::defaultDropAction set to IgnoreAction or CopyAction or LinkAction causes segmentation fault. Is this a desired behaviour?

goetz

If you just copy around pointers it might be that the original object has been deleted already and you work on a dangling pointer then.

A segmentation fault is never a desired behavior :-)

t3chNo

I think it is not desired too. Object is not deleted btw. I'll report this issue.

goetz

Are you sure, it is not deleted? Did you consider using a debugger to look where the segfault pops up? You'll get a nice stack trace that leads you to the misbehaving line of code.

t3chNo

Yes i am sure object is not deleted. Object's pointer is not important. QListWidget doesn't try to access my custom object. It just stores my object's memory address. I made data type to qint64 btw. qint32 is not enough.

goetz

You should use a debugger to see where the segfault happens (access to which object in which place, etc.) We can't help any further on that. It's unlikely to be a bug in the Qt libs as thousands of programmers use this code without any problems.

t3chNo

"bug report":http://bugreports.qt.nokia.com/browse/QTBUG-15265

I added sample code at comment. You can try quickly.

goetz

Nobody can produce an executable from this snippet.

You should create a minimal, complete project to demonstrate the problem.

And could you please tell us if you used a debugger to look where and on which object the segfault happened?

t3chNo

I used debugger and didn't get any useful information. My project that produces this bug: "project":http://rapidshare.com/files/430390643/qlistwidgetitem_test.zip

goetz

Instead of

@
item->setData( Qt::UserRole, QVariant::fromValue( ( QObject* ) new Foo() ) );
@

set your data this way:

@
item->setData( Qt::UserRole, QVariant::fromValue( reinterpret_cast<quintptr>(new Foo()) ) );
@

See the API Docs for a detailed description of "quintptr":http://doc.qt.nokia.com/4.7/qtglobal.html#quintptr-typedef when you get the value back from the item you must cast it back to the original pointer type with

@
reinterpret_cast<Foo *>(value);
@

t3chNo

Sorry for late answer, notification mail didn't come or i didn't see it. Thanks for quintptr, it is more appropriate than qint64. I couldn't find documentation for reinterpret_cast btw. Could you send me link if you know where it is?

goetz

You can find some nice infos on "cplusplus.com":http://www.cplusplus.com/doc/tutorial/typecasting/, also the example is taken from somewhere of the Qt sources, to be honest :-)